The present invention relates to a process and an apparatus for fabricating solid electrolytic capacitors.
JP-B No. 51489/1987, JP-B No. 51491/1987, JP-B No. 66373/1992, U.S. Pat. No. 4,580,855, etc. disclose solid electrolytic capacitors which comprise a capacitor element including a member of valve-action metal and impregnated with a TCNQ complex salt as an electrolyte, the metal member having a chemical conversion coating. By xe2x80x9cTCNQxe2x80x9d is meant 7,7,8,8-tetracyanoquinodimethane.
FIG. 8 shows one type of solid electrolytic capacitor which is already known and has a bottomed tubular aluminum case 9 housing a capacitor element 1, filled with an epoxy resin 91 and having its opening sealed off. As shown in FIG. 9, the capacitor element 1 comprises an anode foil 11 which is an etched aluminum foil provided with a chemical conversion coating, and a cathode foil 12 opposed to and superposed on the anode foil 11 with a separator 13 interposed therebetween. The capacitor element is prepared by rolling up the assembly of the foils and the separator, and impregnating the resulting roll with a solid electrolyte such as a TCNQ complex salt. A pair of lead tab terminals 14, 14 are joined to the anode foil 11 and the cathode foil 12, with lead wires 15, 15 extending from the respective terminals.
The solid electrolytic capacitor described is fabricated by filling a suitable amount of powder of TCNQ salt into a case 9 first, heating the case at 250 to 350xc2x0 C. to melt the salt into a liquid, immersing the roll into the molten salt to impregnate the roll with the salt, thereafter rapidly cooling the roll as placed in the case to solidify the salt and finally filling an epoxy resin 91 into the case 9.
Further attention has been directed to solid electrolytic capacitors which are compact, have a great capacity and are small in equivalent series resistance (ESR) and in which an electrically conductive polymer, such as polypyrrole, polythiophene, polyfuran or polyaniline, is used as an electrolyte. Like the structure shown in FIG. 9, the solid electrolytic capacitor of this type is prepared by rolling up an anode foil 11 provided with a chemical conversion coating and a cathode foil 12 opposed thereto, with a separator 13 interposed between the foils, to obtain a rolled-up element, forming an electrically conductive polymer layer in the rolled-up element to obtain a capacitor element 1, fitting a sealing rubber packing 90 to the capacitor element 1 at one end thereof having projecting lead tab terminals 14, 14, and thereafter placing the element 1 into an aluminum case 9 as shown in FIGS. 6, (a), (b) and (c). The case 9 is then constricted at an opening end portion thereof to hold the packing 90 by the end portion in pressing contact with its periphery to seal off the case 9.
With the conventional process for fabricating the solid electrolytic capacitor wherein the electrolyte is an electrically conductive polymer, a chemical conversion coating, electrically conductive polymer layer, graphite layer and silver paint layer are successively formed on the surface of a sintered anode member or an anode foil of a valve-action metal, such as aluminum or tantalum, and a cathode lead wire is joined to the coated anode member or foil with an electrically conductive adhesive or the like. However, this process of fabrication is considerably more cumbersome than the usual process of fabricating electrolytic capacitors which comprises rolling up an anode foil provided with a chemical conversion coating and a cathode foil opposed thereto, with a paper separator interposed between the foils, and impregnating the resulting rolled-up element (hereinafter referred to as xe2x80x9ccapacitor elementxe2x80x9d) with an electrolyte.
On the other hand, the above-mentioned conductive polymer layer is formed, for example, by electrolytic polymerization or vapor phase polymerization, whereas it is not easy to form the conductive polymer layer in the capacitor element of the rolled-up type by electrolytic polymerization or vapor phase polymerization. Although it appears feasible to form a chemical conversion coating and a conductive polymer layer on an anode foil first and to subsequently roll up the foil along with a cathode foil opposed to the anode foil, difficulty is encountered in rolling up the foils without causing damage to the chemical conversion coating or conductive polymer layer.
The conductive polymer layer can be formed alternatively by chemical polymerization in a liquid phase, whereas this process is conventionally low in work efficiency since it is necessary to repeat five to ten times the procedure of dipping the capacitor element in a chemical polymerization mixture, prepared by diluting with an organic solvent the monomer to be made into the conductive monomer by oxidation polymerization and adding an oxidizer to the solution, and drying the dipped element.
Accordingly, the present applicants developed a process for fabricating a solid electrolytic capacitor comprising a capacitor element including an anode member provided with a chemical conversion coating and an electrically conductive polymer serving as a cathode electrolyte and impregnating the capacitor element, the process having the steps of dissolving an oxidizer in a monomer to be made into the conductive polymer by oxidation polymerization to obtain a mixture and dipping the capacitor element in the mixture (JP-B No. 50558/1998). This process eliminated the need for the repetitions of dipping the capacitor element in the chemical polymerization mixture and the subsequent heat treatment, making it possible to obtain compact solid electrolytic capacitors of large capacity and low ESR by a simple procedure.
FIG. 7 shows the layout of production equipment for practicing the process proposed by the applicants for fabricating the solid electrolytic capacitor. With this equipment, a mixing station 5 and an impregnating station 6 are interconnected by a container conveyor 4, and an element conveyor 3 is disposed along the container conveyor 4. Containers 2 each having a plurality of solution cavities 20 are fed to the mixing station 5, and the monomer to be made into the conductive polymer by oxidation polymerization and an oxidizer (dopant) are placed into the cavities 20 of each container 2 to mix the two solutions together and to thereby initiate the oxidation polymerization of the monomer. The container 2 containing the mixture is thereafter transported toward the impregnating station 6 by the container conveyor 4.
On the other hand, a group 10 of capacitor elements connected to one another is transported by the element conveyor 3 toward the impregnating station 6, at which the capacitor elements are dipped in the mixture in the respective cavities 20 of the container 2.
Consequently, the capacitor elements are impregnated with the mixture of monomer and oxidizer, respectively. The capacitor elements are thereafter allowed to stand in air having a temperature of about 30xc2x0 C. to about 50xc2x0 C. and a humidity of at least 60% for about 30 minutes for the progress of polymerization reaction and further heat-treated in an oven having a temperature of about 160xc2x0 C. for about 5 minutes for drying, whereby a polymer layer is formed over the chemical conversion coating on the anode member.
However, the solid electrolytic capacitors fabricated by the conventional process shown in FIG. 7 have the problem that the electrical characteristics, such as ESR, vary from capacitor to capacitor, failing to provide the desired performance as the case may be.
Accordingly, the present applicants have conducted intensive research to overcome the above problem and consequently found that the mixture of monomer and oxidizer placed in the cavities of the container are not in the form of a uniform mixture at the impregnating station, and that the unevenness of the mixture exerts an influence on the properties of the conductive polymer to be formed over the chemical conversion coating of the anode member to govern the electrical characteristics of the solid electrolytic capacitor as finished, whereby the present invention has been accomplished.
According to the process and apparatus of the invention for fabricating solid electrolytic capacitors, an agitating station is interposed between the mixing station and the impregnating station for agitating the mixture of monomer and oxidizer to render the mixture uniform immediately before the step of impregnating capacitor elements with the mixture.
As a result, a conductive polymer layer of satisfactory properties is always formed on the chemical conversion coating of the anode member in each capacitor element, giving stabilized electrical characteristics to the solid electrolytic capacitor as finished.
The mixture can be agitated by injecting compressed air thereinto with an air injection nozzle provided at the agitating station. Alternatively usable is an ultrasonic generator disposed at the agitating station for applying vibrations to the mixture. Further alternatively, the mixture can be stirred directly with a stirrer disposed at the agitating station.
Thus, the process and apparatus of the invention for fabricating solid electrolytic capacitors provide compact solid electrolytic capacitors of large capacity and low ESR with diminished variations in electrical characteristics.